A 1942 U.S. patent to Wendling (U.S. Pat. No. 2,283,777) shows a greeting card having flaps at its opposite ends. One of the flaps carries a 3-D scene and the other flap carries stereoscopic windows.
U.S. Pat. No. 2,757,573 to Turner and U.S. Pat. No. 4,660,931 to Chevalier disclose viewers for stereoscopic pictures in the form of foldable viewers that may be mailed in flat, relatively compact packages and then opened for viewing. U.S. Pat. No. 4,558,528 to Cunningham discloses a 3-D viewer having a series of scenes. Other patents in the field of this invention that may be of interest to invention historians include U.S. Pat. Nos. 2,986,830, 2,984,153, 906,774, 2,715,853 and 2,696,754. Foreign patents of interest include French 340,661 and European 275.
U.S. patents to Tinker (U.S. Pat. No. 2,616,333), Dennis (U.S. Pat. No. 2,334,483) and Bierstadt (U.S. Pat. No. 174,893) also show stereoscopic devices having some structural similarities to the invention disclosed hereinafter.